This invention relates to in situ recovery of shale oil from formation containing oil shale, and more particularly to the use of rock bolting techniques in unfragmented formation adjacent a horizontal void in a method for forming an in situ oil shale retort.
The presence of large deposits of oil shale in the Rocky Mountain region of the United States has given rise to extensive efforts to develop methods for recovering shale oil from kerogen in the oil shale deposit. It should be noted that the term "oil shale" as used in the industry is in fact a misnomer; it is neither shale, nor does it contain oil. It is a sedimentary formation comprising marlstone deposit with layers containing an organic polymer called "kerogen", which, upon heating, decomposes to produce liquid and gaseous products. It is the formation containing kerogen that is called "oil shale" herein, and the liquid hydrocarbon product is called "shale oil".
A number of methods have been proposed for processing oil shale which involve either first mining the kerogen-bearing shale and processing the shale on the ground surface, or processing the shale in situ. The latter approach is preferable from the standpoint of environmental impact, since the treated shale remains in place, reducing the chance of surface contamination and the requirement for disposal of solid wastes.
The recovery of liquid and gaseous products from oil shale deposits has been described in several patents, such as U.S. Pat. Nos. 3,661,423; 4,043,595; 4,043,596; 4,043,597; and 4,043,598, which are incorporated herein by this reference. These patents describe in situ recovery of liquid and gaseous hydrocarbon materials from a subterranean formation containing oil shale, wherein such formation is explosively expanded to form a stationary, fragmented permeable body or mass of formation particles containing oil shale within the formation, referred to herein as an in situ oil shale retort. Retorting gases are passed through the fragmented mass to convert kerogen contained in the oil shale to liquid and gaseous products, thereby producing retorted oil shale. One method of supplying hot retorting gases used for converting kerogen contained in the oil shale, as described in U.S. Pat. No. 3,661,423, includes establishing a combustion zone in the retort and introducing an oxygen-supplying retort inlet mixture into the retort to advance the combustion zone through the fragmented mass. In the combustion zone, oxygen from the retort inlet mixture is depleted by reaction with hot carbonaceous materials to produce heat, combustion gas, and combusted oil shale. By the continued introduction of the retort inlet mixture into the fragmented mass, the combustion zone is advanced through the fragmented mass in the retort.
The combustion gas and the portion of the retort inlet mixture that does not take part in the combustion process pass through the fragmented mass on the advancing side of the combustion zone to heat the oil shale in a retorting zone to a temperature sufficient to produce kerogen decomposition, called "retorting". Such decomposition in the oil shale produces gaseous and liquid products, including gaseous and liquid hydrocarbon products, and a residual solid carbonaceous material. The liquid products and the gaseous products are cooler by the cooled oil shale fragments in the retort on the advancing side of the retorting zone. The liquid hydrocarbon products, together with water produced in or added to the retort, collect at the bottom of the retort and are withdrawn. An off gas is also withdrawn from the bottom of the retort. Such off gas can include carbon dioxide generated in the combustion zone, gaseous products produced in the retorting zone, carbon dioxide from carbonate decomposition, and any gaseous retort inlet mixture that does not take part in the combustion process. The products of retorting are referred to herein as liquid and gaseous products.
It is desirable to form a fragmented mass having a reasonably uniformly distributed void fraction, i.e., a fragmented mass of reasonably uniform premeability, so that retorting gas can flow generally uniformly through the fragmented mass during retorting operations. Techniques used for excavating void spaces in a retorting site and for explosively expanding formation toward the voids can affect the uniformity of particle size or permeability of the fragmented mass. A fragmented mass having reasonably uniform permeability in horizontal planes across the fragmented mass can avoid bypassing portions of the fragmented mass by retorting gas, which can otherwise occur if there is gas channelling through the fragmented mass owing to non-uniform permeability.
U.S. Pat. No. 4,043,598 discloses a method for explosively expanding formation containing oil shale toward horizontal free faces to form a fragmented mass in an in situ oil shale retort. According to a method disclosed in that patent, a plurality of vertically spaced apart voids of similar horizontal cross-section are initially excavated one above the other at intermediate levels within the retort site. A plurality of vertically spaced apart zones of unfragmented formation are left between adjacent voids, and explosive is placed in blasting holes drilled in the zones of formation and detonated, preferably in a single round, to explosively expand each unfragmented zone into the horizontal voids on either side of it to form a fragmented mass. Retorting of the fragmented mass is then carried out to recover shale oil from the oil shale.
Techniques for expanding roof-supporting pillars along with horizontal zones of formation in a horizontal free face system are described in Application Ser. No. 929,250, filed July 31, 1978, now U.S. Pat. No. 4,192,554 and Application Ser. No. 951,527, filed Dec. 16, 1978 now abandoned. Those applications are owned by the assignee of this application and are incorporated herein by this reference.
In some embodiments it is necessary to leave one or more pillars within each horizontal void for providing temporary roof support for unfragmented formation overlying each void. The pillars can be explosively expanded prior to explosively expanding the unfragmented zones toward the voids. Such prior expansion of the pillars can form a reasonably continuous horizontal free face adjacent such a void. The need to form pillars and the need to explosively expand such pillars in the horizontal voids can contribute to the overall cost and complexity of mine excavation and explosive expansion operations.